The present invention relates to systems and methods for measuring and controlling mass flow and, more specifically, to such systems and methods that allow precise measurement of mass flow using a flow restriction and pressure and temperature sensors.
In many disciplines, the mass flow of a fluid must be measured with a high degree of accuracy. For example, in medical and semi-conductor manufacturing, gasses and liquids often need to be delivered in precise quantities to obtain desired results. Meters are used to measure the mass of the fluid actually delivered.
Conventional pressure-based mass flow meters employ a flow restriction, a temperature sensor, and pressure sensors for detecting the absolute pressure upstream of the flow restriction as well as the differential pressure across the flow restriction. Mass flow is determined from a table that correlates the pressure and temperature readings with predetermined mass flow rates. Such systems require at least two pressure sensors and a temperature sensor to account for fluid density, fluid velocity, and fluid viscosity under different temperatures and upstream and downstream pressures.
The need exists for mass flow meters that are simpler and require less complex calculations to determine true mass flow.
U.S. Pat. No. 5,791,369 to Nishino et al. discloses a flow rate controller that, purportedly, requires only one functional pressure transducer. However, the controller disclosed in the ""369 patent operates only in the sonic flow regime, and this system requires that the inlet pressure be twice the outlet pressure for the controller to function properly. The flow controller of the ""369 patent thus operates only with very low flow rates, only with gases, and must have effective pressure regulation upstream. In addition, the ""369 patent discloses the use of a second pressure transducer to determine when the downstream pressure is more than half of the inlet pressure, and the controller shuts down when this condition is met.
U.S. Pat. No. 6,152,162 to Balazy et al. discloses a fluid flow controller that requires two pressure measurements, one upstream and one downstream of a flow restrictor. The ""162 patent does not measure mass flow. The ""162 patent also employs a filter element as the flow restriction. Particles in the gas stream can clog the filter, thereby changing the relationship of pressure drop and flow characteristics of their flow restriction and possibly deviating from the initial calibration setting.
U.S. Pat. No. 6,138,708 to Waldbusser discloses a pressure compensated mass flow controller. The system described in the ""708 patent combines a thermal mass flow controller with a thermal meter coupled to a dome-loaded pressure regulator. Another pilot pressure regulator using an independent gas source loads the dome of the pressure regulator upstream of the thermal mass flow controller. The pilot regulator and the mass flow controller are controlled by a microprocessor so that inlet pressure is controlled in concert with the flow rate resulting in an inlet pressure independent flow controller.